Material does not react to light and looks shadeless
I figured it out. 


The main problem was that the unit system was set to "none". I set it to "metric" (Context Scene), and then scaled down my object to 0.1. Then the light worked as expected. 
I also had to reduce the strength of the background to 0.2 (Context World).


Is there a good summary of what needs to be considered when switching from 2.79 to 2.8?